A skeleton key (also known as a passingkey) is either a key that has been altered in such a way as to bypass the wards placed inside a warded lock, or a card that contains information necessary to open locks for a certain area like an old abandoned house etc., or an American term for a lever or "bit" type key. The term derives from the fact that the key has been reduced to its essential parts.

The term "skeleton key", in a more general sense, is also sometimes used in reference to a key or similar object capable of opening any lock regardless of make or type. The term refers to the skeletal structure which can bypass all locks. The term is also often misapplied to refer to any antique key.

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A skeleton key is one that has been filed or cut so that it can be used to unlock a variety of warded locks each with a different configuration of wards. This can usually be done by removing most of the centre of the key, allowing it to pass by the wards without interference, unlocking or locking the lock. To counteract this, wards can be placed not just in the center but on the outside as well, making attempts at creating a successful skeleton key harder.

A padlock skeleton key that can open any lock with this keyhole (right), compared to a normal key that can open only the lock for which it was made (left).

A "lever lock" type key.

The term skeleton key is also often used to describe lever lock keys. This usage perhaps stems from such keys' resemblance to a skeletal figure, with the circle at top circumscribing the triangle at one-third the vertical length, resembling a stylized skull.

Lever lock skeleton keys or master keys as they are more commonly known are used in a lock with usually three or five levers and a set of wards that come into contact with the bit of the key only on the sides – the top is for pushing the levers to their correct heights while the warded section of the key just has to pass uninterrupted to allow the key to rotate fully. A master key system will have all the levers heights the same but each door will have different wards. A skeleton key has the warded section of the key removed so that it opens all the doors of a system. Not to be confused with a system where all the locks are keyed alike (one type of key will open all locks but each lock does not have its own unique key that will open only it).

These keys were common in both cabinetry and door locks in early Colonial America and remained in common usage within the United States through the 1940s, giving way after World War II to the pin tumbler lock. Most skeleton keys and their locks were formed of brass, although pewter was sometimes used as well. They were commonly available at hardware and other stores. Today, skeleton keys are associated with fine cabinetry. The types used in cabinetry rarely have double-acting levers and hence tend to be fairly insecure. Skeleton keys are also found in many vending machines, and handcuffs, etc., so that if one key gets lost, the item will still be able to unlock. They are widely used in hotels where customers often tend to misplace their keys or when services like housekeeping need to enter the hotel rooms. These keys are usually responsibilities of top officials because master keys in wrong hands can be destructive.